soc: qcom: pil-femto-modem: Add support for changing Q6 freqs

It is required to be able to select a custom frequency plan for
the Q6 DSPs on the fsm9900 targets.

Change-Id: I3cd9bd3a0f508dc38b87ba801e905eb625c2df72
Acked-by: Jennifer L. Zenner <jzenner@qti.qualcomm.com>
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>